Scanning temporal ultrafast delay and methods and apparatuses therefor

ABSTRACT

The present invention is directed to methods and apparatuses for performing temporal scanning using ultra-short pulsewidth lasers in which only minimal (micro-scale) mechanical movement is required. The invention also relates to methods for obtaining high-accuracy timing calibration, on the order of femtoseconds. A dual laser system is disclosed in which the cavity of one or more of the lasers is dithered, by using a piezoelectric element. A Fabry-Perot etalon is used to generate a sequence of timing pulses used in conjunction with a laser beam produced by the laser having the dithered laser cavity. A correlator correlates a laser pulse from one of the lasers with the sequence of timing pulses to produce a calibrated time scale. The methods and apparatuses of the present invention are applicable to many applications requiring rapid scanning and time calibration, including, but not limited to metrology, characterization of charge dynamics in semiconductors, electro-optic testing of ultrafast electronic and optoelectronic devices, optical time domain reflectometry, and electro-optic sampling oscilloscopes.

This is a continuation of application Ser. No. 10/050,716 filed Jan. 18,2002 now U.S. Pat. No. 7,580,432, which is a Divisional Application ofapplication Ser. No. 08/602,457, filed Feb. 16, 1996 now U.S. Pat. No.5,778,016, which is a Continuation-In-Part of application Ser. No.08/221,516, filed on Apr. 1, 1994 now U.S. Pat. No. 5,585,913. Theentire disclosures of the prior application, application Ser. No.10/050,716 is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is related to the field of ultra-short pulsewidth lasers,and particularly to apparatuses and methods for performing temporalscanning with minimal (i.e., micron-scale) mechanical movement. Theinvention also relates to methods used for obtaining high-accuracy(i.e., sub-picosecond) timing calibration, applicable to theabove-mentioned temporal scanning methods or to conventional temporalscanning methods. In particular, the invention eliminates the need for amechanical scanning delay arm in a correlator or other type ofpump-probe device, including ranging, 3-D imaging, contouring,tomography, and optical time-domain reflectometry (OTDR).

2. Description of the Related Art

Ultrafast laser oscillators are presently known which are capable ofgenerating pulsewidths of the order of tens of femtoseconds withnanojoule-level pulse energies, at repetition rates ranging from 5 MHzto as high as 1 GHz. Such short pulses are used for many applicationsincluding measurements by time gating, including metrology. Manyapplications of such short optical pulses require that one set ofoptical pulses be delayed with respect to another set of optical pulses,in which temporal delays must be known to a very high accuracy, such ason the order of 10 femtoseconds. Temporal delays for short pulses findmany uses in such applications as biological and medical imaging, fastphotodetection and optical sampling, optical time domain reflectometers,and metrology.

The conventional method for delaying and scanning optical pulses is toreflect the pulses from a mirror and to physically move the mirror,using some mechanical means, by a distance D, which is defined by theproduct of the time delay, ΔT, and the speed of light in vacuum c=30×10⁸meters/sec. Thus:D=c/2×ΔT or D (cm)=15×ΔT (nsec).This type of delay will be termed here a physical delay. Also, scanning,as that term is used here, refers to the systematic changing of thedifference in time of arrival between two optical pulses. Variousmethods and devices have been developed to provide the accuratepositioning and scanning of the mirror, involving:

-   -   Voice-coil type devices (shakers) (R. F. Fork Nd F. A. Beisser,        APPL Opt. 17, 3534 (1978)).    -   Rotating mirror pairs (Z. A. Yasa and N. M. Amer, Opt. Comm.,        36, 406 (1981)).    -   Linear translators employing stepper motors, which are        commercially available from many vendors.    -   Linear translators employing galvanometers. (D. C.        Edelstein, R. B. Romney, and M. Scheuermann, Rev. Sci, Instrum.        62, 579 (1990)).        Other types of physical delays use adjustable group delay        including:    -   Femtosecond pulse shapers (FPS) employing scanning galvanometers        (K. F. Kwong, D. Yankelevich, K. C. Chu, J. P. Heritage, and A.        Dienes; “400-Hz mechanical scanning optical delay line” Opt.        Lett. 18, (7) 558 (1993) (hereinafter Kwong et al.); K. C.        Chu, K. Liu, J. P. Heritage, A. Dienes, Conference on Lasers and        Electro-Optics, OSA Tech. Digest Series, Vol. 8, 1994, paper        CThI23.).    -   Rotating glass blocks.

The physical delay methods suffer from a number of disadvantages, thechief one being the large space required if large delays are desired.For example, a delay of 10 nsec requires a mirror displacement of 5feet. There are other physical limitations and disadvantages as well.Misalignment and defocusing can distort measurements when large delaysare used. Using corner-cube retroreflectors reduces the problem ofmisalignment, but not defocusing. The defocusing effect can occur whenthe scan amplitude is an appreciable fraction of the confocal parameterof the beam. A time delay of 10 nsec entails a change in free-spacepropagation of 10 feet (˜3 meters). Thus, to minimize the effects ofdefocusing, the confocal parameter (Z_(R)) must be approximately10-times this number or Z_(R)=30 meters. At a wavelength of 1550 nm,this requires a beam radius of w_(o)=12 mm. This is impracticably largefor many situations.

The need for large mirror displacement can be reduced by multi-passingthe delay line (e.g., double-passing the delay line cuts the requiredmirror displacement in half), however, this does not alleviate thedefocusing problem. Multipassing causes its own set of problems in thatalignment procedures are more complicated, and the optical losses areincreased.

Yet another limitation has to do with the scanning rates and scanningfrequencies which can be achieved simultaneously. It is often desirableto signal average while scanning rapidly (>30 Hz) in order to provide“real-time” displays of the measurement in progress. However, thescanning range is limited at such high scan frequencies. The bestachieved scan range is 100 psec at a rate of 100 Hz using the scanningFPS method (Kwong et al.). Any further increase of the scanning rangeand/or frequency with such reciprocating devices would cause high levelsof vibration, which can be disruptive to laser operation. Rotating glassblocks avert the vibration problem, and are capable of higher scanspeed, but lack any adjustability of scan range, and they introducevariable group velocity dispersion which makes them inappropriate foruse with pulses having widths shorter than 100 fsec.

In addition to the physical delays, methods have been introduced whichprovide temporal scanning without the need for any mechanical motion.These include:

-   -   Free-scanning lasers (A. Black, R. B. Apte, and D. M. Bloom,        Rev. Sci, Instrum. 63, 3191 (1992); K. S. Giboney, S. T.        Allen, M. J. W. Redwell, and J. E. Bowers; “Picosecond        Measurements by Free-Running Electro-Optic Sampling.” IEEE        Photon. Tech. Lett., 6, pp. 1353-5, November 1994; J. D.        Kafka, J. W. Pieterse, and M. L. Watts; “Two-color subpicosecond        optical sampling technique.” Opt. Lett., 17, pp. 1286-9, Sep.        15, 1992 (hereinafter Kafka et al.); M. H. Ober, G. Sucha,        and M. E. Fermann; “Controllable dual-wavelength operation of a        femtosecond neodymium fiber laser,” Opt. Lett. 20, pp. 195-7,        Jan. 15, 1995).    -   Stepped mirror delay lines employing acousto-optic deflectors as        the dispersive elements (R. Payaket, S. Hunter, J. E. Ford, S        Esener; “Programmable ultrashort optical pulse delay using an        acousto-optic deflector.” Appl. Opt., 34, no. 8, pp. 1445-1453,        Mar. 10, 1995).    -   Slewing of RF phase between two modelocked lasers (D. E.        Spence, W. E. Sleat, J. M. Evans, W. Sibbett, and J. D. Kafka;        “Time synchronization measurements between two self-modelocked        Ti:sapphire lasers.” Opt. Comm., 101, pp. 286-296, Aug. 15,        1993).

The non-mechanical methods, in particular, are capable of high speedscanning. The free-scanning lasers produce a scan range which spans theentire repetition period of the laser. For example, a knownfree-scanning laser system is shown in FIG. 1, which includes a masterlaser 10 and a slave laser 20 having different cavity lengths whichproduce pulse trains at different repetition frequencies v₁ and v₂. Thescan frequency is equal to the frequency difference Δv=v₁−v₂, and is setat the desired value by adjusting the cavity length of the slave laserto a specific fixed length. A correlator 40 produces a signal from thecross-correlation between the two lasers which gives information aboutthe timing between the two lasers, and provides triggering signal todata acquisition electronics 50. For example, in Kafka et al. twoindependent, mode-locked Ti:sapphire lasers, namely, master laser 10 andslave laser 20 each with a nominal repetition rate of 80 MHz, were setto have different repetition frequencies (by about 80 Hz). Due to theoffset in repetition frequency, the lasers scanned through each other atan offset frequency Δv of approximately 100 kHz. This offset frequencycan be stabilized to a local RF oscillator. Since the laser repetitionrates were near 80 MHz, the total scan range was about 13 nsec. Thus,time scanning was achieved without employing any moving mechanical delaylines. Timing calibration was achieved by cross-correlating the twolaser beams, reflected off of a mirror 30, in a nonlinear crystal, i.e.,correlator 40, the resulting signal being used to trigger dataacquisition unit 50 (e.g. an oscilloscope). The laser beams output fromlasers 10 and 20 are also reflected off another mirror 60 and receivedby a measurement apparatus 70 which performs a desired measurement orexperiment using the laser beams.

The chief drawback to this technique is that it is highly wasteful ofdata acquisition time for two main reasons:

-   -   1. Fixed scan range—The scan range is fixed at the inverse of        the repetition frequency (i.e., the round trip time) of the        laser.    -   2. Dead time—One is often interested in only a 100 psec, or 10        psec scan range instead of the full 13 nsec pulse spacing. Thus,        only 1% (or 0.1%) of the 10 μsec scan time is useful, while the        remaining 99% (99.9%) is “dead-time.” This increases data        acquisition time by a factor of 100 or 1000.

Kafka et al. address these limitations and suggest that this can bepartially circumvented by using lasers with higher repetition rates,(e.g., v_(o)=1 GHz). However, this solution is unacceptable for manyapplications which require a large variety of scan ranges. For example,pump-probe measurements of semiconductors are frequently conducted overa large variety of time ranges. Lifetimes of carriers (i.e., electronsand holes) of the semiconductor are on the order of several nanosecondswhich makes a 1 GHz laser completely unacceptable, since residualcarriers from the previous laser pulse would still be present when thenext pulse arrives. Yet at the same time, it is often desirable tozoom-in on a much narrower time scale (e.g., 50 psec) to look atextremely fast dynamics. Thus, the free-scanning laser technique lacksthe versatility of scan range selection which is required in manyapplications. The way to get a large temporal dynamic range withouthaving extremely long acquisition times is to have the flexibility of acoarse and fine timing adjustment.

In related work, several methods have been used to stabilize the timingbetween two modelocked lasers, in cases where the lasers were activelymode locked, passively modelocked, and regeneratively modelocked, orwith combinations of passively and actively mode-locked lasers. Themethods used for synchronization can be divided into two main types: (1)passive optical methods; (2) electronic stabilization. The highestsynchronization accuracy is achieved by passive optical methods in whichthe two lasers interact via optical effects (J. M. Evans, D. E. Spence,D. Burns, and W. Sibbett; “Dual-wavelength selfmode-locked Ti:sapphirelasers.” Opt. Lett., 13, pp. 1074-7, Jul. 1, 1993; M. R. X. de Barrosand P. C. Becker; “Two-color synchronously mode-locked femtosecondTi:sapphire laser.” Opt. Lett., 18, pp. 631-3, Apr. 15, 1993; D. R.Dykaar, and S. B. Darak, “Sticky pulses: two-color cross-mode-lockedfemtosecond operation of a single Ti:sapphire laser,” Opt. Lett., 18,pp. 634-7, Apr. 15, 1993 (hereinafter Dykaar et al.); Z. Zhang and T.Yagi, “Dual-wavelength synchronous operation of a mode-lockedTi:sapphire laser based on self-spectrum splitting.” Opt. Lett., 18, pp.2126-8, Dec. 15, 1993). These optical effects, (e.g., cross-phasemodulation) cause a rigid locking effect between the two lasers whichbecome synchronized to within less than one pulse width (<100 fsec).Although these give the most accurate synchronization, the time delaybetween the lasers is rigidly fixed; so that in order to scan the timedelay between them, one must use the conventional physical scanningdelay methods.

Electronic stabilization using simple RF phase detection gives the mostflexibility in terms of adjusting the relative time delay, but at thepresent time these systems can maintain timing accuracy of no betterthan a few picoseconds (˜3 psec). Such a system is commerciallyavailable for stabilizing a Ti:sapphire laser to an external frequencyreference, or for synchronizing two mode-locked Ti:sapphire lasers.(Spectra Physics Lok-to-Clock™ system). Stabilization of better than 100fsec has been achieved by use of a pulsed optical phase locked loop(POPLL). This is a hybrid opto-electronic method, as disclosed in S. P.Dijaili, J. S. Smith, and A. Dienes, “Timing synchronization of apassively mode-locked dye laser using a pulsed optical phase lockedloop.” Appl Phys. Lett., 55, pp. 418-420, July 1989 (hereinafter Dijailiet al.), in which the electronic stabilizer circuit derives the timingerror signal from an optical cross-correlator. However, this methodsuffers from the same lack of timing adjustability as the passiveoptical methods. The timing can only be adjusted by less than one pulsewidth. Thus, using the POPLL method, it would be necessary to insertsome sort of physical delay line into one laser beam if it were desiredto change the relative pulse timing by anything more than onepulsewidth.

The performance of timing stabilization by RF methods could improve ifthe intrinsic timing jitter of the laser can be reduced. Some reductionof intrinsic laser jitter can be achieved by insuring that the twolasers are exposed to identical environmental conditions as much aspossible. The Sticky-Pulse laser, which is disclosed by Dykaar et al.,employs a spatially split pump laser beam to pump two spatially separateregions of a Ti:sapphire laser crystal. This is essentially two separatelasers which share the same pump laser, laser crystal, air space, andmost other intracavity elements except for the two end mirrors. In thisway, the two lasers experience the same thermal fluctuations, pump lasernoise, and turbulence, thus minimizing the difference in repetition ratejitter. This allows even a weak optical interaction between the twolasers to lock the pulses together. This general principle of“environmental coupling” can be applied to other types of lasersincluding mode-locked fiber lasers. However, it should be noted theobject of Dykarr et al. is to lock the two lasers together, which isundesirable for the purposes of the present invention, because then thetime delay cannot be scanned; i.e., the timing of the pulses from thetwo coupled lasers of the Sticky-Pulse laser discussed above, are lockedtogether through the optical coupling and they cannot be independentlycontrolled.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method andapparatus for rapidly scanning the time delay between two modelockedlasers, e.g., “master” and “slave” lasers, without the need for largemechanical movements of optical elements, which would allow scanning ofany selected, contiguous subinterval of the pulse repetition periodT_(R). This is accomplished by making appropriate adjustments, on theorder of microns, to the cavity length of one laser (e.g., the slavelaser), while using an electronic feedback circuit to continuallymonitor the average timing (phase) between the two lasers. Additionally,it is desired to modify the RF stabilization of the two lasers in such away that, unlike the free-scanning laser method, the relative time delaycan be scanned over some sub-interval of the repetition period, thusgreatly improving the duty cycle of data acquisition.

It is another object of the present invention to minimize the timingjitter caused by fluctuations in environmental conditions, such asvibration, air turbulence, and varying temperature. This is accomplishedby constructing both lasers with identical elements within the sameenclosure, and pumping them with the same pump laser, yet allowing themto be independently controlled. In particular, in the case of fiberlasers, it can be accomplished by co-wrapping the fibers together on thesame spool.

It is yet another object of the present invention to provide methods forcalibrating the scanning time scale with subpicosecond accuracy, to beused during and in conjunction with the scanning method mentioned above.For example, the present invention can be used to cross-correlate thepulses from one laser (e.g., a master laser) with pulse sequencesobtained by passing pulses from the other laser (e.g. a slave laser)through a series of partially reflective optical elements.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and advantages of the invention will becomesmore apparent and more readily appreciated from the following detaileddescription of the presently preferred embodiments of the inventiontaken in conjunction with the accompanying drawings, of which:

FIG. 1 shows a known free-scanning laser system;

FIGS. 2(A) and (B) show a fast scanning laser system according to thepresent invention, in which the cavity length of one or both of themaster and slave lasers is controlled;

FIGS. 3(A) and (B) show time delay T_(D)(t) and waveforms applied to thePZT in the laser cavity;

FIG. 4 shows a twin fiber laser system according to the presentinvention, having two identical mode-locked fibers lasers which usenonlinear polarization evolution as the mode-locking mechanism, in whichfibers of both lasers are co-wrapped on the same spool;

FIGS. 5(A) and (B) show a plot of a cross-correlation signal between twofiber lasers scanned by the dithering method of the present invention,shown in FIG. 4, the sinusoidal voltage applied to the PZT, and thesquare wave trigger-out of the signal generator;

FIG. 5(C) shows a plot of timing jitter data;

FIGS. 6(A) and (B) show a high-finesse Fabry-Perot etalon producingpulse trains;

FIGS. 7(A)-(D) show plots of two full scan cycles including thecross-correlation signals between a single laser pulse and the pulsetrain transmitted through an air-spaced PP etalon;

FIGS. 8(A) and (B) show a pulse train generator consisting of a seriesof photorefractively-grown chirped fiber gratings;

FIG. 9 is a detailed schematic diagram of a laser stabilization anddithering system;

FIG. 10 is a schematic diagram of a measurement system employing thefast scanning lasers and the timing calibration method according to thepresent invention;

FIGS. 11(A) and (B) show metrology systems which employ a fast scanninglaser system according to the present invention;

FIG. 12 shows a femtosecond OTDR system employing a fast scanning lasersystem and timing calibration method according to the present invention;and

FIG. 13 shows an electro-optic sampling oscilloscope employing a fastscanning laser system and timing calibration method according to thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments of the present invention are described below in moredetail with reference to the above drawings, in which like referencenumerals refer to the same or similar elements.

Principle of Fast Scanning Method

The present invention, like the free-scanning laser system, consists oftwo lasers, a master laser 110 and a slave laser 120, which have nearlyidentical repetition rates as shown, for example, in FIG. 2(A). However,unlike the free-scanning laser system, pulses output from the master andslave lasers are not allowed to totally scan through each other. Rather,while master laser 110, which has a wavelength λ1, is held at a constantrepetition rate v₁ or is allowed to drift of its own accord, therepetition frequency v₂ of slave laser 120 is dithered about therepetition rate of master laser 110. This dithering of the repetitionrate is accomplished by changing the cavity length (L₂) of slave laser120 at a “high” frequency, for example, in the range of 30 Hz to 1 kHz,while its average repetition rate is slaved, or controlled, to that ofmaster laser 110 by a stabilizer unit 130 which includes a “slow” phaselocked loop (PLL) circuit whose bandwidth is less than the scanfrequency. The average timing delay between the master and slave lasersis held constant by stabilizer unit 130 which controls the cavity lengthof slave laser 120. Meanwhile, a fast dithering signal output from asignal generator 140 is summed with the control voltage output fromstabilizer unit 130 to scan the instantaneous delay between the lasers.The slaving and dithering of the repetition frequency can be implementedby mounting an end mirror of slave laser 120 on a piezoelectrictransducer (PZT) 121 and applying the requisite voltage signal from afrequency signal generator 140. It is required that the PLL circuitbandwidth be less than the dithering frequency; otherwise, it will tryto track the master laser and counteract the cavity length scanning.

FIG. 2(B) shows another embodiment in which master laser 110 includes aPZT 111, and both the master and slave lasers 110 and 120 have mirrorscontrolled by PZTs 111 and 121. In this case, master laser 110 isdithered at the scan frequency while slave laser 120 tracks the averagerepetition rate of master laser 110. The cavity length of master laser110 is dithered rapidly by signal generator 140 while the stabilizerlocks slave laser 120 to the desired average time delay.

Again, it is necessary for the dithering frequency to exceed thebandwidth of the PLL circuit in order that the PLL not counteract thetime scanning.

As an example of how the cavity length scanning operates, a square waveis applied to slave laser PZT 121 at a scanning frequency of f_(s). Thenthe cavity length mismatch is a rapidly varying function of time:ΔL(t)=ΔL ₀ ·Sq(f _(s) t)  Eq. 1where ΔL₀ is the amplitude of the square wave displacement, and Sq(x) isthe square wave function. This would give positive and negative linearscanning delays for one half of the scan cycle (a triangle wave). In thetime stationary case, a constant cavity length mismatch of ΔL producesan offset frequency of:

$\begin{matrix}{{\Delta\; v} = {- \frac{c\;\Delta\; L}{2L^{2}}}} & {{{Eq}.\mspace{14mu} 2}a}\end{matrix}$which can be rewritten as

$\begin{matrix}{\frac{\Delta\; v}{v} = {- {\frac{\Delta\; L}{L}.}}} & {{{Eq}.\mspace{14mu} 2}b}\end{matrix}$However, in the fast scanning method, the cavity length is dithered at afrequency high enough so that the pulses never have a chance tocompletely walk through each other. That is, the scan frequency andamplitude satisfy the condition

$\begin{matrix}{{f_{s} ⪢ {\Delta\; v}}{or}{f_{s} ⪢ \frac{c\;\Delta\; L}{2L^{2}}}} & {{Eq}.\mspace{14mu} 3}\end{matrix}$In this case, the time-varying time delay T_(d)(t) would be proportionalto the time integral of the cavity length mismatch:

$\begin{matrix}{{{T_{d}(t)} = {\frac{2}{{cT}_{1}}{\int_{0}^{t}{\Delta\;{L\left( t^{\prime} \right)}\ {\mathbb{d}t^{\prime}}}}}}{or}{{T_{d}(t)} = {\frac{1}{L}{\int_{0}^{t}{\Delta\;{L\left( t^{\prime} \right)}\ {\mathbb{d}t^{\prime}}}}}}} & {{Eq}.\mspace{14mu} 4}\end{matrix}$where c is the speed of light in vacuum, and the period of integrationis on the order of one scan cycle. FIG. 3(A) shows an instance ofsquare-wave modulation, in which a 1 kHz square wave is applied to thePZT in one of the lasers. The waveforms shown in FIG. 3 show atime-dependent cavity length mismatch, ΔL(t), and the resultinginstantaneous time delay, T_(D)(t), as a function of time when variousdithering waveforms are applied to the PZT in the slave laser. Theinstantaneous repetition-rate v₂ dithers around v₁, and the relativetime delay scans back and forth linearly in time. The total scan rangeis dependent on the base repetition rate v₁ the scan frequency f_(s),and the scan amplitude ΔL, according to the equations:

$\begin{matrix}{T_{\max} = {\frac{\Delta\; L}{2L} \cdot \frac{1}{f_{s}}}} & {{{Eq}.\mspace{14mu} 5}a}\end{matrix}$or, in terms of the offset frequency:

$\begin{matrix}{T_{\max} = {\frac{1}{2}{\frac{\Delta\; v}{v} \cdot \frac{1}{f_{s}}}}} & {{{Eq}.\mspace{14mu} 5}b}\end{matrix}$The scan rate, in units of (msec/msec) is given by:

$\begin{matrix}{{R_{scan} = {2\;\Delta\;{{Lv}/c}}}{or}{R_{scan} = \frac{\Delta\; L}{L}}} & {{{Eq}.\mspace{14mu} 6}a}\end{matrix}$or, in more convenient laboratory units:

$\begin{matrix}{R_{scan} = {10^{9}\frac{\Delta\; L}{L}\mspace{14mu}{\left( {{psec}\text{/}{msec}} \right).}}} & {{{Eq}.\mspace{14mu} 6}b}\end{matrix}$The “sampling grid” is given by the temporal pulse advance per roundtrip:δt _(g)=2ΔL/c  Eq. 7.A scanning velocity parameter can be defined, which is the amount ofspatial pulse advance per second. This is given by

$\begin{matrix}{{v_{scan}(t)} = {\frac{2\Delta\;{L(t)}}{T_{R}} = {\frac{\Delta\;{L(t)}}{L} \cdot {c.}}}} & {{Eq}.\mspace{14mu} 8}\end{matrix}$Note that these concepts of scan rate, sampling grid, and scan velocityapply equally well to free-scanning lasers.

As an example, a pair of lasers with a nominal cavity length of L=1.5meters gives v₁=100 MHz. If a scan amplitude of ΔL=15 μm is assumed, anoffset frequency of Δv=1 kHz, a scan rate of R_(scan)=10⁴ psec/msec, anda sampling grid of δt_(g)=100 fsec result. Full pulse walkoff occurs in1 msec if the cavity length is not dithered. Thus, a scan frequency off_(s)>1 kHz is required to prevent total walkoff if a 15 μm scanamplitude is used.

Tables 1, 2, and 3 show some possible scan ranges, scan rates, andsampling grid intervals as a function of various scan parameters, formodelocked lasers with repetition rates v of 10 MHz, 100 MHz, and 1 GHz.It can be seen from Table 3 that for a 1 GHz laser, most practical scanconditions produce total walkoff, hence, the time delay cannot exceed 1nsec. Thus, for lasers with repetition rates under 1 GHz, the ditheringmethod of the present invention is preferable to the free-scanning lasermethods.

TABLE 1 Scan Parameters for a 10 MHz Laser (L = 15 m). Scan Scan Freq.ΔL₀ Offset Freq. Scan rate Grid ScanVel. range f_(s) (Hz) (μm) Δν (Hz)(psec/msec) (fsec) (m/sec) (psec) 10 0.15 1 10 1 3 50 10 1.50 10 100 1030 500 10 15.00 100 1000 100 300 5000 100 0.15 1 10 1 3 5 100 1.50 10100 10 30 50 100 15.00 100 1000 100 300 500 1000 0.15 1 10 1 3 0.5 10001.50 10 100 10 30 5 1000 15.00 100 1000 100 300 50

TABLE 2 Scan Parameters for a 100 Mhz Laser (L = 1.5 m). Scan Scan Freq.ΔL₀ Offset Freq. Scan rate Grid ScanVel. range f_(s) (kHz) (μm) Δν (Hz)(psec/msec) (fsec) (m/sec) (psec) 0.1 0.15 10 100 1 30 500 0.1 1.50 1001000 10 300 5000 1.0 0.15 10 100 1 30 50 1.0 1.50 100 1000 10 300 5001.0 15.00 1000 10,000 100 3000 5000 10.0 0.15 10 100 1 30 5 10.0 1.50100 1000 10 300 50 10.0 15.00 1000 10,000 100 3000 500 100.0 1.50 1001000 10 300 5 100.0 15.00 1000 10,000 100 3000 50

TABLE 3 Scan Parameters for a 1 Ghz Laser (L = .15 m) Scan Scan Freq.ΔL₀ Offset Freq. Scan rate Grid ScanVel. range f_(s) (kHz) (μm) Δν (Hz)(psec/msec) (fsec) (m/sec) (psec) 0.1 0.15 1 1000 1 300 * 0.1 1.50 1010,000 10 3000 * 1 0.15 1 1000 1 300 500 1 1.50 10 10,000 10 3000 * 115.00 100 100,000 100 30,000 * 10 0.15 1 1000 1 300 * 10 1.50 10 10,00010 3000 500 10 15.00 100 100,000 100 30,000 * 100 0.15 1 1000 1 300  5100 1.50 10 10,000 10 3000  50 100 15.00 100 100,000 100 30,000 500 *Total walkoff (1 nsec maximum)

Symmetric square wave dithering is not the only desirable method. FIGS.3(B) and (C) show two other dithering schemes in addition to the squarewave modulation. An asymmetric type of rectangular wave can be used inorder to obtain unidirectional scanning, as shown in FIG. 3(B). Thiswould reduce the wasted dead-time of the backward scan. Whileunidirectional scanning is used in commercial Fabry-Perotinterferometers, it is produced by a triangular waveform rather then bya rectangular waveform, as shown in FIG. 3B.

The abrupt voltage changes on the leading and falling edges of therectangular and square waves can cause mechanical shock and ringing inthe PZT, thus distorting the otherwise linear time scale of scanning.Some type of signal conditioning (e.g., smoothing of transitions) whichwould reduce these effects is desirable. Alternatively, a sinusoidalvoltage can be applied to the PZT, as shown in FIG. 3(C). The advantageof sinusoidal scanning is that it avoids the shock and ringing whichwould accompany the sharp voltage transients associated with the squareand rectangular waves. It also avoids timing distortion as the PLL triesto react to the induced cavity length mismatch. That is, for a simpleanalog stabilizer circuit, a sinusoidal drive voltage will produce asinusoidal scanning characteristic, even if the stabilizer reactssomewhat to the scanning time delay. For these reasons, sinusoidalscanning is the simplest to implement. However, sinusoidal scanning doesnot share the advantage of a rectangular wave signal, such as that shownin FIG. 3B, which produces unidirectional scanning. Therefore, dependingon the degree of simplicity required, a sinusoidal scanning signal canbe desirable for driving the PZT.

The scan parameters given in Tables 1-3 were provided based on symmetricsquare wave modulation of the PZT. For sinusoidal scanning, the scanrange and scan rate would be modified somewhat. The time-dependent delayis still given by equation 4, however, the cavity length modulation isgiven by:ΔL(t)=ΔL ₀·cos(2πf _(s) t)  Eq. 9.Then the delay is:

$\begin{matrix}{{T_{D}(t)} = {\frac{\Delta\; L_{0}}{L} \cdot \frac{1}{2\pi\; f_{s}} \cdot {{\sin\left( {2\pi\; f_{s}t} \right)}.}}} & {{Eq}.\mspace{14mu} 10}\end{matrix}$This will give sinusoidal scanning, although the time delay will be 90degrees out of phase with the PZT position. Note that the scanningcharacteristic is not linear, and some type of scale correction isneeded.

It should be noted that although the laser scanning technique describedhere was demonstrated using a pair of mode-locked fiber lasers, thistechnique is not limited to the embodiments described above, but isapplicable to many types of mode-locked lasers including solid-state,diode lasers, and dye lasers.

Scanning Fiber Laser System

FIG. 4 shows a preferred embodiment of the invention for a twin fiberlaser system employing two fiber lasers, i.e., a master laser 210 and aslave laser 220.

Both lasers are pumped by the same laser diode LD 205, whose power isdivided between the two lasers by a splitter SPL 206. Master laser 210is terminated by an integrated Faraday rotator mirror FRM 215, whileslave laser 220 is terminated by an optical assembly PZT-FRM 225 whichis equivalent to FRM 215, but which also contains a mirror mounted on aPZT. The two fiber lasers contain identical modelocking optics (i.e.,waveplates λ/4, λ/2, Faraday rotators (FR), and polarizing beamsplittersPBS). Laser mode-locking is initiated by saturable absorbers SA. Theoutput pulses from the low-noise output ports of the lasers are detectedby photodiodes PD-1 and PD-2 which are used to drive stabilizer 130.

Both fiber lasers have a nominal repetition rate of v₀=4.629 MHz. Theyare based on an environmentally stable design disclosed in M. E.Fermann, L. M. Yang, M. L. Stock, and M. J. Andrejco, “Environmentallystable Kerr-type mode-locked erbium fiber laser producing 360-fspulses.” Opt. Lett., 19, pp. 43-5, January 1994, which uses nonlinearpolarization evolution (NPE) as the mode-locking mechanism. The lasershave identical components except for the Faraday rotator mirrors; masterlaser 210 is terminated with an integrated Faraday rotator mirror FRM215 which is a single package, while slave laser 220 is terminated byassembly PZT-FRM 225 of discrete components which is equivalent to aFRM, but which also contains a mirror mounted on a PZT. The PZT usedhere had a total travel range of 40 microns. The lasers are thermallyand mechanically coupled by co-wrapping them on the same fiber spool280. Additionally, both lasers are pumped by the same pump laser diodeLD 205 so that the pump noise in the two lasers is correlated. Therelative timing between the lasers is set and stabilized by stabilizer130, which includes a PLL circuit. Once stabilizer 130 has been enabledand the lasers are set at the proper delay, a dithering signal isapplied to the PZT of slave laser 220 to make it scan. The ditheringsignal is applied by a signal generator 135, which is added to astabilizing signal output from stabilizer 130 to produce a signal fordriving the PZT. Alternatively, stabilizer 130 can generate thedithering signal, so that the output of stabilizer 130 includes both thestabilizing and dithering signals. Each laser has two output portsassociated with its intracavity polarizer. These two output ports areshown in FIG. 4 as the two outputs of each PBS. These two laser outputshave very different noise characteristics; one is somewhat noisy, andthe other has very little noise, due to an optical limiting effect (dueto the NPE) which is known to occur in this type of laser. It is thisquiet output beam that is detected by photodiodes PD-1 and PD-2 whichprovide input to stabilizer circuit 130. Use of the quiet output beam asthe input to stabilizer 130 helps to minimize the timing jitter.

Sinusoidal time scanning has been accomplished by the inventors by usingthe twin fiber laser system. FIG. 5(A) shows 2 cycles of across-correlation scan between the two fiber lasers shown in FIG. 4,collected with single shot data acquisition, that is, with no averaging.More particularly, FIG. 5(A) shows the cross-correlation signal betweentwo fiber lasers being scanned by the dithering method of the presentinvention at a scan frequency of 106 Hz and over a scan range of 200psec. Here, FIG. 5(A) shows two full cycles of scan, showing bothforward and backward scans. Also shown is the sine wave voltage appliedto the PZT controller, and the trigger out of the signal generator,which appears as a square wave. This plot of FIG. 5(A) was obtained bypassing the pulses from the two lasers through a modifiedcross-correlator which uses sum frequency mixing in a nonlinear crystal,in particular a beta-barium borate (BBO) crystal.

A scanning mechanical delay is used in one arm of a known correlator.However, in the correlator of the present invention, no mechanical delaywas used. All scanning was done using the laser dithering methoddescribed above. Also, FIG. 5(A) shows the sine wave voltage applied tothe PZT controller and the trigger-out of the function generator. Notethat the ends of travel of the PZT which are marked on the graph are 90degrees out of phase with the applied sine wave, as expected accordingto equations 9 and 10. The scanning range is about 200 psec at a scanfrequency of 100 Hz. This is equivalent to scanning a physical delay by3 cm at a repetition rate of 100 Hz. However, with the twin-laser systemthe same scanning range is achieved by moving the PZT of the slave laserby only a few microns.

This cross-correlation method was used to measure the timing jitterbetween the two lasers. The timing calibration was provided by insertinga glass etalon of having a thickness of 2 mm into one arm of thecorrelator, which creates a sequence of pulses separated by 20 psec.These sequence of pulses are clearly visible on the scans shown in FIG.5(A). FIG. 5( b) shows the same scan on an expanded time scale showingclosely spaced pulses (one laser has satellite pulses) and replicas ofthe pulse pairs, produced by inserting a 2 mm glass etalon into one armof the correlator. The pulse pair replicas are separated from the mainpulse pair by 20 psec, corresponding to the optical thickness of theetalon. Here, it can be seen that the pulse widths are on the order of 1psec, and satellite pulses are also present a few psec away from themain pulse. The RMS timing jitter thus measured was ΔT_(j)=5 psec, withoccasional timing deviations of up to ±20 psec. The timing jitter dataare shown in FIG. 5(C) where each data point represents the relativetime delay between the lasers on each scan at the 106 Hz scan rate.

The measured jitter demonstrates the limitations of the precision of theelectronic PLL circuit of stabilizer 130, but it also demonstrates howprecise timing information can be obtained in spite of this jitter. Ifthe scan is done rapidly enough, the relative timing jitter within thescan time can be quite small. And if a stable series of timing pulses isprovided, e.g., by passing laser pulses through an etalon, then even thejitter during the scan time can be known accurately. In this way, thescan characteristic can be known to subpicosecond accuracy even thoughthe lasers are jittering by several picoseconds.

The beneficial effects of co-packaging the lasers is important. In aprevious, and less successful dual laser system built by the inventors,the same pair of lasers were constructed separately on differentbreadboards and were pumped by different lasers. With the stabilizerenabled, the slave laser would track the master laser for only about 30minutes before the cavity length mismatch would exceed the 40 microntravel range of the PZT. At this point, tracking became impossible.Thus, even under a normal room temperature environment, the cavitylength mismatch between a pair of 5 MHz lasers can drift by more than 40microns, which is beyond the range of most PZTs. On the other hand, thecopackaged system of the present invention is able to trackindefinitely, indicating that the cavity length mismatch stays wellwithin the 40 micron limit of the PZT under normal room conditions.Measurements of the absolute and relative frequency drifts show that therelative frequency drift between the two lasers is 7-times less than theabsolute drift of one laser. The relative drift can be improved stillfurther by making the lasers truly identical in construction. This canbe accomplished by terminating both lasers with identical PZT-FRMassemblies, or by terminating them both with identical FRM packages andchanging the slave laser cavity length with a fiber stretcher.

Further reduction of both the relative and absolute timing drifts can beaccomplished by acoustic damping of fiber spool 280 and othercomponents, and building the lasers in the same enclosure, which coulditself be acoustically damped and temperature controlled. Eventually,with all of the above methods employed, relative laser jitter couldreach the quantum limit (H. A. Haus and A. Mecozzi, “Noise ofmode-locked lasers,” IEEE J. Quantum Electron., QE-29, pp. 983-996,March 1993). Since the quantum-limited timing jitter increases withincreasing dispersion, the jitter could be further reduced by operatingthe mode-locked lasers near the zero-dispersion wavelength.

Dithering the end mirror of a mode-locked laser can induce amplitudefluctuations at the scan frequency due to mirror misalignment anddefocusing. The misalignment effect can be minimized by focusing on thePZT-mirror to reduce angular sensitivity, and by using a 3-pointmirror-scanning PZT to preserve alignment, as is done with commercialscanning Fabry Perot interferometers. The defocusing effect can occurwhen the scan amplitude is an appreciable fraction of the confocalparameter of the beam waist incident on the PZT-mirror. In a fiberlaser, this defocusing causes a reduction in the coupling efficiency ofthe beam back into the fiber, which in turn, causes power fluctuations.Thus, tight focusing on the PZT mirror is undesirable. A judiciouschoice, of beam collimation can reduce the defocusing effect. Forexample, if a PZT is used having a scan range of 40 microns, a confocalparameter of at least a few millimeters would be required. Then, thequantity (ΔL/Z_(R))²˜10⁻⁴ is quite small, where Z_(R) is the confocalparameter of the beam waist at the PZT mirror. When this quantity issmall, the amplitude modulation of the laser is correspondingly small.

It should be pointed out that even if slight mirror misalignments causesome small amplitude fluctuations during the scanning, the beam pointingstability is not degraded in any way, due to the guiding properties ofthe fiber. However, with a solid-state modelocked laser, some deviationof the output beam could occur if measures are not taken to prevent it.

An alternative laser system might have both lasers terminated byidentical FRM, and the cavity lengths could be adjusted by a PZT fiberstretcher, such as a piezoceramic tube actuator PiT 40×18×1 made byPiezomechanik GmbH of Germany.

Timing Calibration

Even with the PLL circuit of stabilizer 130 stabilizing the averagerelative timing delay, this position can fluctuate by severalpicoseconds so that it is necessary to insure that the data acquisitionunit 50 is properly triggered with a timing signal which has the desiredaccuracy, i.e. 100 fsec. Such a signal can be obtained from across-correlator employing nonlinear optical mixing in a nonlinearcrystal such as BBO, as was done in the demonstration mentioned above,and by others, such as Kafka et al. This type of triggering has alsobeen shown to be essential in obtaining high accuracy in metrologymeasurements, even when using a mirror shaker for the scanning timedelay. However, while this is necessary, it may not be sufficient.Jitter measurements show that the timing can fluctuate not only fromscan to scan, but even during a single scan cycle. That is, the scanrate R_(scan), changes from scan to scan, and even during the scancycle. It is therefore essential that time scaling operations occurbefore signal averaging. Thus, it is necessary to have at least twotiming pulses per scan from the correlator—one for triggering, and theother for time scale information. To do this, the present inventionfills the scanning interval with a sequence of pulses, either uniform ornonuniform (in both amplitude and time). In this way, timing informationcan be obtained about the scanning interval on each and every scancycle.

Timing Scale Generation

An important key point of the time scale calibration is selecting theoptical methods used to generate the timing information. In a preferredembodiment, a sequence of pulses can be produced by reflecting a singlepulse from a high-finesse Fabry-Perot (FP) etalon to produce atemporally uniform sequence of many pulses, as shown in FIGS. 6(A) and(B). Because the FP etalon is being used as a “rattle-plate”, in whichthe pulse is internally reflected many times within the FP etalon (i.e.,a high-Finesse FP etalon), rather than its resonant properties beingused, the transmission through it is rather low. An example of employingan FP etalon consisting of two mirrors with R=98%, thus having atransmission of T=0.0004, is now discussed. The transmitted pulse train,shown in FIG. 6(A), is a series of pulses, separated by the transit timeof the FP etalon, ringing down by a decay factor which is dependent onthe mirror losses and the misalignment. That is, the transmitted pulsetrain is attenuated by the factor T² where T is the transmission of theetalon surfaces. The transmitted pulse train is more or less uniformwith the pulse intensities slowly decaying upon each round trip. Thefirst (and largest) pulse in the train is 2500-times weaker than thepulse incident on the FP etalon. FIGS. 7(A) and (B) show the slowlydecaying pulse train transmitted through a real air-spaced FP etalonwith surface reflectivities of R=98%, and mirror spacing ofapproximately 1 mm. FIG. 7(B) shows, on an expanded time scale, a plotof a single backward scan having 17 pulses and showing the pointcorresponding to the end-of-scan. These data were obtained by simplyinserting the FP etalon into one arm of a conventional scanningcorrelator which employed a retro-reflector mounted on a voice-coil(speaker). Since a sinusoidal voltage was applied to the speaker, thetime delay was scanned in a more or less sinusoidal fashion. Thespacings of the pulses versus time give the scanning characteristic ofthe shaker-mirror, which is shown in FIG. 7(C). The total scan range isreadily calculated from these data, as:T _(max)≈(17 pulses)×(6.7 psec/pulse)=114 psec  Eq. 11.

The data in FIG. 7(C) show that, as expected, the scanningcharacteristic is not exactly linear. To show the deviation fromlinearity, the data is fitted with a straight line, and then the data issubtracted from this best fit. The result, shown in FIG. 7(D), is thedeviation of the time scanning characteristic from linearity.

The reflected pulse train is identical to the transmitted pulse train,which are each shown in FIG. 6(B) except that the reflected pulse trainis preceded by a prompt pulse which is the first surface reflection andwhich contains most of the pulse energy (i.e., 98%). Since this firstreflected pulse is at nearly full intensity, it is useful for any of theexperimental applications or measurements to be performed. Note thatthis prompt pulse is 2500-times more intense than the pulse trainimmediately following it. In most cases, this weak pulse train will notseriously affect the measurements. In other cases, however, it may beunacceptable.

Of course, etalons with lower finesse can also be used, but then thetransmitted pulse train decays much more quickly, and the correlator'sdynamic range becomes the limiting factor. For example, an etalon withsurface reflectivity of R=30% produces a rapidly decaying series ofpulses in which each pulse is 10-times weaker than the previous pulse.Thus, only about 3 pulses can be used for calibration in real time,since most data acquisition has a single shot dynamic range of onlythree decades. This was the case for the correlation shown in FIG. 5.This effect can be overcome to some extent by dynamic range compressionschemes, for example, by using a logarithmic amplifier.

In yet another embodiment, combinations of FP etalons can be used. Forexample, a thin etalon (such as a cover-slip) would produce a pair ofclosely spaced pulses (˜1 psec) which would then be sent to a higherfinesse FP with a larger spacing (e.g., 20 psec). The result would be asequence of pulse pairs spaced by 20 psec. This gives local derivativeinformation about the timing characteristic.

The FP etalons could be either solid or air-spaced. A solid etalon ismore rugged and compact, but an air-spaced etalon is adjustable andavoids the problem of pulse broadening due to group velocity dispersionwhich would occur for those pulses which have made many round-tripsthrough the etalon. For high accuracy, it is necessary to control thetemperature of the etalon. For example, the fractional change in groupdelay through a piece of fused silica is approximately Δl/l˜10⁻⁶/° C.Thus, if 1 micron (6 fsec) of accuracy is required over an entire rangeof 1 meter (6 nsec), then the etalon temperature should be constant towithin 1° C. Air-spaced etalons have an advantage in that they can beconstructed using thermally compensating mounting techniques. Thus, thetemperature sensitivity can be reduced over that of solid etalons, andtemperature control may not be required for normal room-temperatureoperation.

Another possible implementation is to use a series of photorefractivelygrown fiber gratings 310, as shown in FIG. 8(A), grown on an opticalfiber 300. The gratings can be so tailored that the emerging pulsesequence can be either uniform or nonuniform. In this case, the spacingis uniform, but the amplitudes are nonuniform in order to produce a“ruler” scale, with larger pulses occurring for every 5th or 10th pulse,as shown in the figure. Of course, the ability to grow fiber gratings atany desired position in the fiber gives great flexibility, and it may beadvantageous to place them at nonuniform spacings in order to eliminatetiming ambiguity.

Additionally, if it is desired that the pulses traverse substantiallengths of fiber (i.e., >0.5 meters) the pulses will broaden due togroup velocity dispersion (GVD) in the fiber. Then, a series of chirpedgratings could be grown in such a way that the GVD of the fiber beingtraversed is properly compensated. In fact, chirped gratings arenecessary for high time resolution due to the fact that normal unchirpedgratings have reflectance bandwidths of only about 2 nm making themunsuitable for subpicosecond pulses. At a center wavelength of 800 nm, a100 fsec pulse has a full-width half-maximum (FWHM) bandwidth of 8 nm;and at a wavelength of 1500 nm, the bandwidth is about 30 nm. Onlychirped gratings have sufficiently broad bandwidths for these pulses. Itis known that reflecting femtosecond pulses from chirped gratings causesthe pulses themselves to become broadened and chirped. In this case, itis necessary to sequentially reflect the pulses from two chirpedgratings with opposite senses of chirp, as is schematically illustratedin FIG. 8(B). Here, pulse broadening is prevented by using acompensating scheme in which the pulse is first reflected from a chirpedgrating sequence CFGS 320 and subsequently reflected from a nearlyidentical chirped grating CFG 330 which is chirped in the oppositesense. The order can be reversed (i.e., reflect first from CFG and thenfrom CFGS) with equal results. Efficient splitting and reflection can beprovided by polarizing beamsplitter PBS 340 and by quarter-wave platesQWP 350. This technique has been successfully applied toward chirpedpulse amplification in fibers by A. Galvanauskas, M. E. Fermann, D.Harter, K. Sugden, and I. Bennion, “All-fiber femtosecond pulseamplification circuit using chirped Bragg gratings.” Appl Phys. Lett.,66, pp. 1053-5, Feb. 27, 1995. The object of this aspect of theinvention, however, differs from that of Galvanauskas et al. In thatwork, a pair of gratings were used to stretch the optical pulse to verylong durations (>300 psec) for chirped pulse amplification, and then torecompress the pulses. In the present invention, chirped gratings areused because unchirped gratings have insufficient bandwidth (˜1 nm) tosupport short pulses, and because there will be some need to compensatethe GVD of the fiber for pulses which have traversed substantial lengthsof fiber. Here, there is no desire to stretch the pulses, because theobject of this aspect of the invention is to produce a sequence of shortpulses.

Two fiber gratings can also be used to form a fiber FP etalon, whichcould be used either in transmission or in reflection. Alternatively, apassive optical fiber loop could be used. Other structures which canproduce reflections are poor splices between optical fibers andmicrobends in a fiber. Temperature control of the fiber would benecessary in these cases, or variations in timing calibration would beknown beforehand, and compensated for mathematically.

Pulse sequences can also be produced by a series of partially reflectingmirrors, with the reflectivity and spacing of each mirror beingcarefully chosen so as to give the desired sequence of pulses, if it isphysically possible. Algorithms have been developed for calculating therequired mirror parameters to produce the desired pulse sequences from asingle pulse (V. Narayan et al., “Design of multimirror structures forhigh-frequency bursts and codes of ultrashort pulses,” IEEE J. QuantumElectron. QE-30, pp. 1671-1680, July 1994). There are a number of othertypes of optical devices, composed of partial reflectors, which can beused as well to generate pulse trains from a single pulse.

Arbitrary, programmable pulse sequences can be produced by passing alaser pulse through a pulse-shaping dispersive delay line (DDL). Thisgives much greater flexibility than the methods mentioned above, but islimited in terms of the maximum achievable pulse spacing. Practicallimits are about 100-200 psec. Any larger pulse spacing requires aphysically larger setup, which can become prohibitive.

Another embodiment, most useful for low repetition rate (≦30 MHz) lasersat wavelengths near 1550 nm, would be to inject a sequence of pulsesinto a regenerative soliton storage ring consisting of a fiber loop witha gain section (e.g. Er-doped fiber). The pulse sequence injected intothe ring may be obtained in any number of ways including the methodsdiscussed here (i.e., Fabry-Perot etalon, fiber gratings, pulse shaper,etc.). Because the loop is regenerative, it is necessary to dump it andre-inject it on every laser pulse. This would occur every 200 nsec for a5 MHz laser. The dumping and injection can be accomplished by some typeof optical switch, either AO or EO, which are commercially available.

Yet, another embodiment would be to inject short pulses from one laserinto a laser diode biased at or near threshold. The laser diode facetshave a reflectivity of 30%, forming a low-finesse FP etalon. However,the gain of the laser diode would retard or prevent the “ringdown” ofthe optical pulses. In this way, it should be possible to obtain a trainof several tens of pulses. Eventually, group velocity dispersion andgain narrowing in the diode would broaden pulses which make many roundtrips through the structure, and thus limit the number of pulses whichare actually usable. This device could be used either in reflection ortransmission.

Extremely fine (sub-micron) calibration can be obtained by inserting abirefringent crystal (e.g. quartz) into one beam of the measurementsystem and comparing the pulse arrival times when the pulses propagatealong the ordinary and extraordinary axes of the crystal. Typical platethicknesses of 1 mm give a few wavelengths of retardation, whichproduces a readily discernable difference in pulse arrival times ifpulses of 100 fsec duration or less are used.

Cross-Correlator Optics

The sequence of pulse signals generated according to the above-describedmethods are cross-correlated with the other laser pulses using some sortof nonlinear element. Various nonlinear processes can be used fordetection of pulse coincidence or relative pulse timing. Some opticalnonlinearities which could be used include, but are not limited to:optical mixing via second harmonic generation (SHG), sum frequencygeneration, gain saturation, absorption saturation, four-wave mixing(FWM), and nonlinear response of photocurrents. A likely choice for thenonlinear element is a SHG crystal. However, using a SHG crystal has thedisadvantage of only providing a signal when there is overlap betweenthe laser pulses. Thus, it can serve only as a coincidence detector.This requires the use of the various pulse train generators, pulseshapers, etalons, . . . etc., discussed above. In some situations, theextremely high accuracy which can be obtained using SHG crystals is notrequired. In some cases it may be more desirable to use an element whichhas a noninstantaneous response, and thus, can give information abouttime delay via amplitude information. There are a number of devices ormaterials which might be suitable, such as traveling-wave laser diodeamplifiers (TWAs); saturable absorbers; photodetectors, which rely onsaturation effects, such as PIN diodes and avalanche photodiodes (APDs);or perhaps even SEED devices. For example, TWA's have been used foroptical clock recovery in optical communications. They have theadvantage that when they are driven far into saturation, they becomerelatively immune to amplitude fluctuations. Here, the accuracy andrange are determined by the recovery time of the device. TWA devices areknown to measure phase accuracy to about 10⁻³ radians. In suchoptoelectonic devices, nonlinearity of optical absorption is not theonly method which can provide timing information. Pulse timinginformation can also be obtained by monitoring electrical propertiessuch as photocurrent, voltage, capacitance, etc., of the device aspulses propagate through them. This would be a significantsimplification over detecting the change in optical properties.

These timing calibration techniques, although intended for use with thefast-scanning laser method, are expected to be applicable not only tothe laser scanning systems described here, but also to free-scanninglasers, dual-wavelength mode-locked lasers, and even to the moreconventional scanning systems employing physical delays. An example ofthis is the calibration data shown in FIG. 7 which was obtained for ascanning system which consisted of a retro-reflector mounted on avoice-coil (speaker). Since a sinusoidal voltage was applied to thespeaker, the time delay was scanned in a more or less sinusoidalfashion. The scanning characteristic shown in FIG. 7(C) can be used tocorrect the time scale of any data acquired during the scan. When theactuator is driven at higher amplitudes, near its physical limits, thescanning characteristic deviates significantly from a sinusoid. This canbe corrected as well. Other devices, such as rotating glass blocks androtating mirrors, also have nonlinear scan characteristics which can becorrected by the timing methods of the present invention, describedhere.

Laser Stabilization

Because any cavity length error is constantly accumulated on each roundtrip of the laser cavity, very small cavity length fluctuations cancause large timing errors. Thus, it is necessary to use a servo loop tohold the time-averaged repetition rate to v₁; or equivalently, thetime-averaged cavity length mismatch, ΔL, must be held to zero. Thefeedback signal which can be used to control the average cavity mismatchis derived from a pair of photodetectors PD-1 and PD-2 shown in FIG. 9,which feed into a conventional phase locked loop (PLL) circuit. Asdiscussed earlier, the accuracy of such a PLL stabilizer system, hasbeen measured and been found able to synchronize the two fiber lasers towithin 5 psec RMS with maximum timing excursions of up to 20 psec. Thequoted stabilization accuracy is the current state of the art, and by nomeans represents the ultimate attainable limit. It should be possible tosubstantially increase the accuracy to well below 1 psec. However, it isnot expected that the stabilization accuracy will ever approach thedesired time resolution, which in some cases could be 1 fsec or evensmaller. For this reason, the timing calibration methods described herewill still be required.

A detailed diagram of the preferred embodiment of the laserstabilization and dithering system, including stabilizer 130, is shownin FIG. 9. Stabilizer 130 includes timing discriminators TD 131 and 132,a phase detector 133, a filter 134, a DC voltage generator 135, anamplifier 136, a frequency generator 140 and an adder 137. Pulseamplifiers (PA) 430 and 440 receive the electrical pulses output fromPD-1 and PD-2, respectively. The pulse amplifiers 430 and 440 amplifythe received electrical pulses and output them to timing discriminators131 and 132, respectively. Timing discriminators 131 and 132 conditionthe signals before they are input to the phase detector 133. The PZTcontroller used here is a commercial PZT controller unit. This is ahigh-voltage amplifier which takes an input signal in the range of 1-10volts, and produces a proportional output in the range of 0-150 volts.Although the PZT controller unit is shown as a separate unit, it couldbe, and preferably is, integrated into stabilizer 130.

In a preferred embodiment of the stabilizer 130, the phase detector 133for the PLL can be a standard RF phase detector, or mixer, or a logicgate, such as an XOR gate, which has improved linearity. The linearityis very important if it is desired to perform scanning of anysubinterval of the round trip time. Alternatively, a time-to-amplitudeconverter (TAC) can be used as the phase detector. This is also verylinear, and would be especially appropriate at low repetition rates,such as 5 MHz. One limitation to highly accurate phase stability is theAM to FM conversion which can occur in a simple RF mixer. That is, laseramplitude fluctuations are converted to timing fluctuations by themixer. It is thus desirable to perform signal conditioning on theelectrical pulses which are produced by the photodiodes PD-1 and PD-2.This can be accomplished most readily by using timing discriminators 131and 132 before phase detector 133, as shown in FIG. 9. However, thiseffect can also be minimized by reducing the amplitude noise of thelasers. It is known that an optical limiting process occurs duringmode-locked operation under certain conditions, and for certain outputports of the laser. This greatly reduces the timing jitter when thestabilizer is enabled.

It is also important that the scanning frequency be greater than thebandwidth of the stabilizer system. In this way, the PLL would maintainthe average time position properly, but would not counteract the appliedmirror scanning. The twin fiber lasers described earlier were stabilizedusing a PLL circuit which had a bandwidth of only 30 Hz. This rather lowbandwidth is advantageous for allowing scan rates ranging from 30 Hz upto several kHz. However, in order to be able to use a 30 Hz stabilizerbandwidth and still maintain synchronization with <10 psec accuracy, theintrinsic relative timing jitter of the laser pair must be very low.This is achieved using the methods of construction described earlier,which ensure that the two lasers are subjected to the same environmentalconditions to the greatest extent possible.

It is also possible that a simple correlator could provide the necessaryfeedback signal for stabilization. While the laser is scanning, the peakposition, as measured by the correlator, could be used as an errorsignal to feed back to stabilizer 130. Note that the simple stationarycross-correlator technique of Dijaili et al. will not work here, sincein the present invention the lasers are constantly scanning.

The fast scanning system of the present invention has many advantagesover conventional scanning physical delays and free-scanning lasers. Inparticular, unlike the conventional scanning physical delays, which havemoving arms, no misalignment or defocusing of the laser beams occurs inthe present invention even when scanning large delays of several nsec.With physical delays, very careful alignment must be performed, and theconfocal parameter must be >1 meter for even a 1 nsec delay line.Furthermore, high scanning speed, and even hypersonic scan velocitiesare possible with the present invention. However, with a physical delay,scanning even 1 nsec at 100 Hz (100 feet/sec) is not practical.Although, with the present invention, large (adjustable) scanning rangesare possible, for example, (from ˜50 psec up to 200 nsec for a 5 MHzfiber laser). To achieve such a scanning range in a conventionalscanning system, 200 feet of delay line would be required. Also, thereis no need to employ a high repetition rate laser for better duty cycle,as in conventional free scanning systems. Further, large temporaldynamic range is possible, which would be useful for scanning of remotetargets or OTDR. For example, a total scan range of T_(R)=200 nsec withtime resolution less than 1 fsec gives a temporal dynamic range of morethan 10⁸. Also, an improved duty cycle with greatly reduced dead-time isachieved, unlike the conventional free scanning system. Also, extremelysimple and compact cross-correlator design is now possible with thepresent invention, without any moving parts. For example, the correlatorcould be the size of a Game-Boy. Moreover, experimental setups aregreatly simplified because pathlengths do not need to be matched in thepresent invention.

The fast scanning and timing calibration methods discussed above can beused to perform many types of measurements and experiments. A few of theapplications in which the methods and apparatuses of the presentinvention can be used, are discussed below. However, a person skilled inthe art will readily understand that the present invention is applicableto many applications and is not limited to the following applicationsdescribed below.

FIG. 10 shows the preferred embodiment of a general measurement systememploying the fast-scanning laser system and the timing calibrationmethod using a FP etalon, according to the present invention. Pulsesfrom laser 510 (laser 510 may be either the master or slave laser, itdoes not matter which) are incident on a FP etalon. The transmittedpulse train transmitted through the FP etalon is sent to a timing unit540 in which it is cross-correlated with a pulse from laser 520, togenerate a data stream which gives a calibrated time scale. The pulsetrain reflected from the etalon is sent to a measurement unit (notshown) along with a pulse from laser 520. The data stream from themeasurement unit is input into the “Y-channel” of the data acquisitionsystem (DAQ) 550 while the data stream from the timing unit is sent tothe “X-channel” of DAQ 550. This information can be used in twodifferent ways.

1. On-the-fly (OTF) time scale correction—In this technique, the timingpulses form a time scale. For example, if sinusoidal scanning is used,then even a uniform pulse sequence will appear non-uniform in time. Fastprocessors could use this time scale information on each scan toappropriately adjust the scanning data (e.g., by using interpolation)with the correct time scale before signal averaging.

In other words, each scanning point is corrected by the fast processorbased on the time scale formed by the timing pulses. For example, FIG.7(C) shows the scanning characteristic for each peak of the scan shownin FIG. 7(B), and FIG. 7(D) shows the linear deviation of these points.This time scale information can be used to correct for the deviations,in essence calibrating each peak “on-the-fly.”

2. Scan rejection (smart triggering)—This technique uses a“scan-selector” which looks for a number of timing pulses (at least two)to occur within certain well-defined time slots relative to the triggerpulse. The selector then makes a GO/NO-GO decision on whether to sum thescan data into the signal averager. If the timing pulses from thecorrelator fall within the time slots, the scan is summed into theexisting data buffer; if one of the timing pulses “misses” its timingslot, the scan is rejected. After signal averaging is completed, anynonlinearity in the time scale can be properly compensated.

The smart triggering method is the simplest to implement; however, manyscans would be wasted. The on-the-fly scale correction is moresophisticated and more time efficient in terms of wasted scans, but ismore computationally intensive. Accordingly, the constraints of theapplication will likely determine which technique to use.

Metrology System

As an example of a more specific application, FIGS. 11(A) and (B) showpreferred embodiments of a metrology system which employ the fast laserscanning technique of the present invention instead of the conventionalmoving-mirror method.

FIG. 11(A) shows an embodiment of a metrology system. Here, the beamfrom slave laser 620 is incident on the FP etalon which produces asequence of pulses which are input to timing unit 641. The beam frommaster laser 610 is split by a normal beamsplitter BS 660. One part ofthe split beam of the slave laser 620 is input to timing unit 641. Theother part of the beam is directed to the surface under test, whichreflects the beam. The reflected beam is input to correlator 640. Also,the pulse reflected from FP etalon is input to correlator 640. Thus,correlator 640 is used for the object (experiment) to be measured, andtiming unit 641 is used for the time scale calibration. The data streamcorresponding to the object which is output from correlator 640, isinput to the Y-channel of DAQ 650, and the data stream output fromtiming unit 641, which also includes a correlator, goes to the X-channelof DAQ 650. The timing unit data input to the X-channel provides a timescale, and the data output from correlator 640 and input to theY-channel give object/surface distance information.

FIG. 11(B) shows another embodiment of a metrology system employing thetechniques of the present invention. Here, the timing device (FP etalon)is put into a beam path to the object. The beam reflected from the FPetalon illuminates the object to measured. The light scattered from theobject surface is collected by a lens L1, and is then recombined (via abeamsplitter BS 670) with the calibrated pulse train which istransmitted through the FP. The resulting cross-correlation between agating pulse from slave laser 620 and this series of pulses from thetarget and etalon, gives the object range information (a single pulse)superimposed on the multi-pulse timing scale from the etalon. The resultis a single data stream which contains both the object range informationand the time scale calibration. The distance to the point on the objectsurface is then deduced by measuring the relative time delay between theobject pulse and the timing pulses in the data stream. This comparisonbetween the timing of the object pulse and the etalon pulses is avariation of a differential metrology method using ultrashort pulses.

In FIGS. 11(A) and (B), the FP etalons are shown to be oriented at asharp angle with respect to the input beam. This angle of incidence hasbeen exaggerated for clarity. Those skilled in the art will recognizethat tilting the FP etalon away from normal incidence will causeincreasing lateral displacements of each subsequent reflection from theetalon, as shown in FIG. 6. Therefore, it is desirable to use the etalonat an angle of incidence which is as small as is practical. It would beused at normal incidence if it is preceded by a polarizer and Faradayrotator. This spatial displacement will not degrade the correlatorperformance as long as it does not overfill the lens or any otheraperture in the correlator.

Alternatively, this lateral displacement effect could be used to someadvantage in modifying the shape of the pulse train envelope to besomething other than a decaying exponential. This could occur viaaperturing of the beams, or by the angular selectivity of the phasematching condition of the nonlinear mixing crystal in the correlator.

OTDR System

FIG. 12 is a schematic diagram of an optical time domain reflectometer(OTDR) system employing the fast scanning laser and timing systems ofthe present invention. The scanning laser system preferably is comprisedof low repetition rate lasers (v˜5-10 MHz) so that the usableunambiguous range is large. A short pulse from a master laser 710 issplit into two beams by beam splitter BS 760. One of the split beams issent to a timing unit 741, and the other is sent to a fiber or waveguidedevice under test (DUT) 790. Pulses reflected from surfaces, splices,defects, etc. in the DUT 790 are sent to a correlator 740 for precisiontiming/distance measurement with accuracies of ˜10 fsec/˜3 microns. Thisis accomplished according to the following method. Single pulses fromslave laser 720 are split into two beams. One of the beams is sent tocorrelator 740 to be used as a gating pulse for the signal from thefiber. The other is sent to a pulse shaper 780 which produces pulsesequences which are in turn used by a timing unit 741, which is itselfanother correlator, to provide a calibrated time scale for the dataacquisition (DAQ) unit 750. For a low repetition rate system such as 5MHz (T_(R)=200 nsec), pulse shaper 780 must produce pulse sequenceswhich more or less fill the entire 200 nsec timing interval. In thepreferred embodiment, the pulse shaper would utilize chirped fiber Bragggratings, as shown in FIG. 8(B).

Other configurations of the laser, timing units and correlators arepossible. For example, the roles of the master and slave lasers 710 and720, could be interchanged. The pulse shaper could also consist of afiber FP etalon, or a fiber loop (either passive or with gain).

EO Sampling Oscilloscope

FIG. 13 shows a schematic diagram of a jitter-free electro-opticsampling oscilloscope using the fast scanning laser and timing systemaccording to the present invention. This embodiment combines the wellestablished non-contact EO-sample technique with the fast scanningtechnique described herein, thus providing much greater flexibility intime scale adjustment.

Pulses from master laser 810 are split into two beams. One beam is sentto a timing unit 841; the other pulse is sent to generate electricalpulses on the device under test (DUT), which in this case is mounted onan integrated circuit (IC) 890, via photo-conductive (PC) switch 891.Pulses from slave laser 820 are also split into two beams. One beam issent to the electro-optic (EO) probe tip 892, and the other beam is sentto the pulse shaper 880 which produces a sequence of pulses, which inturn are sent to timing unit 841 for time scale calibration. The pulsesreturned from the EO probe tip 892 are modulated by the interactionbetween voltage from the DUT and the EO probe tip 892. These pulses aredetected by polarizing optics 893 and sent to a Y-channel of DAQ 850.Accordingly, precision timing calibration is obtained in a similarmanner as described for the OTDR of FIG. 12.

The timing delay, scanning interval, and scanning frequency are set tothe desired values by the stabilizer 830. For example, if it is desiredto increase the scanning frequency by a factor of two (but not the scanrange), then it is necessary to double the modulation frequency of thePZT in slave laser 820, and to increase the PZT scanning voltage by acorresponding amount. On the other hand, to increase the scan rangewithout changing the scan frequency, one simply increases the PZTscanning voltage. The relative delay of the time sweep range is adjustedby the phase control of stabilizer 830. This flexibility of time scaleadjustment is similar to that afforded by the delaying time base of aconventional oscilloscope.

One could equally well use photoconductive sampling for this purpose.This has much higher sensitivity, however, the time resolution islimited to ˜2 psec.

The most widely used applications of short laser pulses and scanningdelays have been in pump-probe measurements of physical, chemical, &electronic systems. However, the range of applications for thistechnology is increasing rapidly, and their introduction into commercialproducts beyond the scientific market, is imminent. The rapid scanningand time calibration techniques described here can be used in almost anyapplication which utilizes ultrafast laser pulses, because virtually allof these applications require some type of adjustable time delay betweenlaser pulses. These applications include, but are not limited to,electo-optic testing of ultrafast electronic and optoelectronic devicescharacterization of charge dynamics in semiconductor materials anddevices, all-optical signal processing photoconductive sampling, andvarious forms of time-resolved, nanometer probing. Additionally, thereare a number of new applications being developed for terahertz beams towhich these scanning methods could be applied including terahertzimaging, such as that disclosed by B. B. Hu et al., “Imaging withTerahertz Waves”, Optics Letters, Vol. 20, No. 16, Aug. 15, 1995, pp.1716-1719. Recently, ultrafast photodetectors have become commerciallyavailable, whose speed (50 GHz, 10 psec FWHM) far outstrips themeasurement capability of commercial sampling oscilloscopes, such asNewport Corp. Model #PX-D7, manufactured by Picometrix. Subpicosecondlasers and scanning delays are required to take full advantage of thespeed of these detectors. Furthermore, the scanning method of thepresent invention, because of its unusual versatility of scanning rangeselection, is especially appropriate for submillimeter-resolution laserradar, and profiling of remote targets. The flexibility of the scanrange adjustments makes the fast-scanning system analogous to theadjustable time base on an oscilloscope.

The very high scan rate achievable with the fast-scanning methodsdescribed here, makes possible many new potential applications. The scanvelocity parameter (see Tables 1-3) is a figure of merit which isparticularly useful. Note that the scan velocities in Tables 1-3 rangefrom 3 m/sec to 30,000 m/sec. Thus, supersonic scan velocities arepossible with the free-scanning or fast scanning techniques, making thempotentially useful for studies of, or applications involving,laser-induced acoustic and photoelastic effects in solids and liquids,for example. Such applications are not practical using conventionalscanning methods because it would be necessary to move the scanningmirror at sonic velocities in a physical delay system, which isunrealistic.

The present invention has been described in connection with thepreferred embodiments, and is not intended to be limited only to theabove-described embodiments. Other modifications and variations to theinvention will be apparent to those skilled in the art from theforegoing disclosure and teachings. Thus, while only certain embodimentsof the invention have been specifically described herein, it will beapparent that numerous modifications may be made thereto withoutdeparting from the spirit and scope of the invention.

1. A short-pulse laser, comprising: a fiber laser for generating a pulseoutput; and an acoustically damped fiber spool, around which said fiberlaser is wrapped to stabilize the output of said fiber laser.
 2. A fiberlaser system comprising: a first rare-earth doped fiber operable toconduct optical energy; and a spool around which said first fiber iswrapped, wherein said first rare-earth doped fiber is isolated fromexternal environmental conditions in an isolation device for limitingthe absolute timing drift and for stabilizing a repetition rate of saidfiber laser system.
 3. The fiber laser system as claimed in claim 2,further comprising: a second rare-earth doped fiber co-wrapped aroundsaid spool; and a single optical pump source operable to drive both saidfirst and second rare-earth doped fibers.
 4. The fiber laser system asclaimed in claim 3, further comprising: a first Faraday rotator mirrorat an end of said first rare-earth doped fiber; an optical assemblycomprising a second Faraday rotator mirror and a piezoelectrictransducer mounted on a mirror.
 5. The fiber laser system as claimed inclaim 3, wherein the fiber laser system comprises at least first andsecond mode locked fiber lasers, the system further comprising: at leasttwo identical sets of modelocking optics, each set of modelocking opticscomprising a waveplate, a Faraday rotator and a polarizablebeamsplitter, wherein at least one set of modelocking optics isassociated with said first rare-earth doped fiber and at least one otherset of modelocking optics is associated with said rare-earth dopedsecond fiber.
 6. The fiber laser system as claimed in claim 2, whereinsaid rare-earth doped fiber is disposed in a fiber laser and mode lockedto produce short optical pulses, and wherein changing the repetitionrate of pulses output from said fiber laser is accomplished by changingthe length of fiber laser cavity.
 7. The fiber laser system as claimedin claim 6, wherein the length of said laser cavity is changed in apredetermined manner by applying a voltage signal to a piezoelectricunit disposed within a fiber laser cavity.
 8. The fiber laser system asclaimed in claim 7, wherein said voltage signal comprises a square wave,a rectangular wave, or a sinusoidal wave.
 9. The fiber laser system asclaimed in claim 6, wherein the length of said laser cavity is changedin a predetermined manner by applying a voltage signal to apiezoelectric unit disposed within the laser cavity.
 10. The fiber lasersystem as claimed in claim 6, wherein said short optical pulses compriseat least one pulse having a pulse width less than 100 fs.
 11. The fiberlaser system as claimed in claim 2, wherein said isolation devicecomprises at least a temperature controlled enclosure.
 12. A fiber lasersystem comprising: a first rare-earth doped fiber operable to conductoptical energy; a spool around which said first fiber is wrapped, asecond rare-earth doped fiber co-wrapped around said spool; and a singleoptical pump source operable to drive both said first and secondrare-earth doped fibers, wherein said first rare-earth doped fiber andsaid second rare-earth doped fiber are isolated from externalenvironmental conditions in an isolation device for limiting theabsolute timing drift and for stabilizing a repetition rate of saidfiber laser system.
 13. The fiber laser system as claimed in claim 12,wherein said isolation device comprises at least a temperaturecontrolled enclosure.
 14. The fiber laser system as claimed in claim 12,wherein said first and second rare-earth doped fibers of said fiberlaser system are mode locked to produce respective first and secondshort optical pulses from respective first and second mode locked fiberlasers, and wherein changing the repetition rate of pulses output from afiber laser is accomplished by changing the length of a laser cavityhaving a rare-earth doped fiber.
 15. The fiber laser system as claimedin claim 14, wherein said first and second short optical pulses compriseat least one pulse having a pulse width less than 100 fs.
 16. The fiberlaser system as claimed in claim 14, wherein the length of said lasercavity is changed in a predetermined manner by applying a voltage signalto a piezoelectric unit disposed within a fiber laser cavity.